In the prior art, in order to charge a capacitor of a discharge type welding tool, charge current is controlled by a resistor which produces undesired power consumption, and heats the internal portions of the tool which increases the temperature therein, resulting in undesired operation of circuits and damage to the circuit elements. When a high charge potential is required, charging time becomes long which in turn increases the working time. Excessive charging current flows at the initial stage of the charging to burn the electrical elements such as the power transfomer.